The present invention relates to a method for measuring the moisture content of skin, in which method a probe is placed on the skin for measuring the capacitance of the skin. The invention relates also to the apparatus for applying the method, which apparatus comprises a probe to be placed on the skin and devices connected to the probe for transmitting and transferring a signal into the probe and back from the probe in order to transfer and analyse the reflected signal and for measuring the capacitance of the skin.
The skin consists of three layers, which are, from the surface inwards, stratum corneum, epidermis and dermis. The water content is low in the stratum corneum and high in the dermis. The water content increases gradually from the surface of epidermis to deeper parts of epidermis. The hydration of skin is determined by the properties of the stratum corneum. Dry skin feels rough and will become wrinkled faster than normal skin. People living in dry weather conditions suffer from dry skin more than others and have to apply cosmetic products for moisturizing the skin.
There exist devices for skin moisture measurement used for the assessment of skin condition. These devices can also be used for evaluating the efficacy of skin moisturizing products. Innumerable devices have been developed for the measurement of parameters sensitive to the moisture of skin. Some of the devices are expensive and complicated to use, but some are relatively cheap and easy to use, e.g. ohmmeters measuring the electric resistance of the skin. However, they have many shortcomings: the distance between the electrodes varies and the devices are sensitive to the electrolyte content of the intercellular space and to the skin contact.
The principle of the present moisture meters based on the measurement of the skin capacitance is that the skin is a capacitive part of a resonance circuit, either an RC-circuit or a monostabile multivibrator. A device containing an RC-circuit is introduced in the U.S. Pat. No. 4,860,753. A monostabile multivibrator is introduced in the U.S. Pat. No. 4,711,244. The skin causes a change in the resonance frequency of the circuit. The resonance frequency that indicates the properties of the skin is for instance calculated with a counter. One weakness of these devices is that the frequency of the measurement circuit depends on the properties of the skin. However, the dielectric properties of the skin change in relation to the frequency due to the influence of different types of dispersions. The dielectric properties determine the capacitance of the skin, which is the reason why a device measuring capacitance is not working reliably when its frequency changes. Another weakness of mentioned devices is that they are sensitive also to the resistivity of the skin, which means that, for example, the relative content of the electrolytes, which is independent of the water content of the stratum corneum, affects the measurement result.
The object of the present invention is to provide a method and an apparatus, which obviate the shortcomings of the present methods and apparatuses. It is a particular object of the invention to provide a method and an apparatus for measuring the moisture content of skin reliably, and that the resistivity of the skin does not affect the measurement result. It is a further object of the invention to provide an apparatus which is simple in its structure and economical to manufacture and to use.
In the method in accordance with the invention a wave signal is transmitted to the probe, the capacitance of the probe is measured by comparing the phases of the transmitted and reflected wave, and the applied radio frequency is low, approximately 0.2 to 5 MHz. This comparison can be performed with an appropriate device known as such.
This method enables accurate measurement of the capacitance of the probe even in the case when the conductance of the probe is much higher than the susceptance of the probe, which occurs always when the probe is in contact with the skin. In this arrangement the measurement result is independent of the conductance of the probe, which in practice means that the measurement result does not depend on the resistivity and further, for example, on the electrolyte content of the skin.
Operating frequency of the apparatus is an essential parameter of the method in accordance with the invention. The range used in the method is approximately 0.2 to 5 MHz. With lower frequencies than this, the susceptance of the probe decreases further and the difference between the conductance and the susceptance increases further. On the other hand, with frequencies higher than the specified range the measurement result contains information also from deeper layers of the skin. This is undesirable, because the water content of dermis does not affect what is commonly perceived as the moistness of the skin.
In an advantageous application of the invention the apparatus operates at only one frequency, the low radio frequency, approximately 0.2 to 5 MHz. When this method is used the apparatus operates only at one precisely set frequency, and thus gives a reliable result of the moisture content of skin. The capacitance of the skin is determined by its dielectric properties and therefore only such capacitance measuring apparatus which operates with a constant frequency, can be a reliable apparatus.
In a further advantageous application of the invention a wave signal is transmitted to the probe by an oscillator, and placing an attenuator on the signal path between the oscillator and the probe prohibits the reflections of the waves forming the signal between the oscillator and the probe. This means that the reflections do not affect the measurement result.
In a further application of the invention the phases of the wave transmitted to and reflected from the probe are deviated with a power splitter from each other by 90 degrees in addition to the minor phase difference caused by the probe. The resolution of the phase detector is then optimal and the accuracy of the measurement is better than when using a normal zero-degree power splitter.
In an additional application the phases of the wave transmitted to and reflected from the probe are compared by a phase detector. The phase detector is dependable and reliable but in other applications other devices can be used for this purpose.
Furthermore, in an advantageous additional application the propagating waves are amplified in the phase detector inputs so that the phase detector operates in a saturated state. Only then the phase detector or a similar device measures merely the phase shift of the incoming signals.
An apparatus in accordance with the invention comprises a device, preferably a phase detector, into the input of which the wave formed by the signal reflected from the probe is arranged to be led, and a wave forming the signal is arranged to be led into the other input of the device, then the capacitance can be measured by comparing the wave phases.